1. Field of the Invention
This invention relates to a driving circuit for an electrostatic recording head used in electrostatic recording such as facsimile or other electrostatic recording machines.
2. Description of the Prior Art
A multistylus head 1 having control electrodes P.sub.1, P.sub.2, P.sub.3, . . . , P'.sub.1, P'.sub.2, P'.sub.3, . . . , on the same plane as the recording pin electrodes 3 is shown in FIG. 1; this type of recording head is generally used in electrostatic recording machines. In this type of head 1, a row 3 of a plurality of pin electrodes is disposed on one plane of a base 2. A first and second row of control electrodes P.sub.1, P.sub.2, P.sub.3 . . . and P.sub.1', P.sub.2', P.sub.3'. . . are arranged facing each other with pin electrode row 3 therebetween. The second row of control electrodes is positioned opposite the first row in order to create a uniform electric field for recording. Driving circuits are utilized for generating high voltage recording pulses which are coupled to the control and pin electrodes.
FIG. 2 is a conventional driving circuit for an electrostatic recording head. The second row of control electrodes is omitted from the following explanation. It should be noted, however, that each electrode of the second control electrode row is simultaneously impressed with the same voltage as its corresponding control electrode within the first row.
As shown in FIG. 2, the pin electrodes are separated into sequential even and odd arrays, each array consisting of four pin electrodes. As shown, the electrodes form pin electrode arrays A.sub.1, B.sub.1, . . . , A.sub.k, B.sub.k, . . . Arrays A.sub.1, A.sub.2, . . . , A.sub.k . . . are odd arrays, while arrays B.sub.1, B.sub.2, . . . B.sub.k . . . are even arrays. The odd arrays and the even arrays are connected to pin electrode driving circuits 4a and 4b, respectively. Facing pin electrode arrays A.sub.1, B.sub.1 are control electrodes P.sub.1, P.sub.2 ; P.sub.2, P.sub.3, facing pin electrodes A.sub.k, B.sub.k are control electrodes P.sub.2k-1, P.sub.2k ; P.sub.2k, P.sub.2k+1. The control electrodes are connected to a control electrode driving circuit 5 for energizing the control electrode corresponding to a desired pin electrode.
When recording voltages corresponding to the recording picture signals are impressed on arrays A.sub.1, . . . , A.sub.k, . . . by pin electrode driving circuit 4a, voltages of an opposite polarity are impressed on control electrodes P.sub.2k-1, P.sub.2k by control electrode driving circuit 5. As a result, electrostatic latent images are formed on the recording medium just below the odd array. When voltages corresponding to the recording picture signals are impressed on arrays B.sub.1, . . . , B.sub.k, . . . by pin electrode driving circuit 4b, voltages of an opposite polarity are impressed on control electrodes P.sub.2k, P.sub.2k+1 by control electrode driving circuit 5. As a result, electrostatic latent images are formed on the recording medium just below the even array. Various methods of driving multistylus heads which have control electrodes on the same plane can be employed which determine the sequence of recording. As will be discussed, prior art methods provide many disavantages. One prior art method is shown in Table 1.
TABLE 1 ______________________________________ Array of Recording Voltage Impressed Positions Control Electrodes ______________________________________ A.sub.1 P.sub.1, P.sub.2 B.sub.1 P.sub.2, P.sub.3 A.sub.2 P.sub.3, P.sub.4 B.sub.2 P.sub.4, P.sub.5 . . . . . . A.sub.k- 1 P.sub.2k- 3, P.sub.2k- 2 B.sub.k- 1 P.sub.2k- 2, P.sub.2k- 1 A.sub.k P.sub.2k- 1, P.sub.2k B.sub.k P.sub.2k, P.sub.2k+ 1 . . . . . . ______________________________________
According to this driving method, voltages are first impressed on both control electrodes P.sub.1 and P.sub.2, then on both control electrodes P.sub.2 and P.sub.3 and successively to control electrodes P.sub.2k and P.sub.2k+1. Simultaneously, voltages are alternately impressed on pin electrode arrays A.sub.1, A.sub.2, . . . , A.sub.k, . . . , and B.sub.1, B.sub.2, . . . , B.sub.k, . . . by respective pin electrode driving circuits 4a and 4b. As a result, recording successively occurs under arrays A.sub.1, A.sub.2, B.sub.1, B.sub.2, . . . , A.sub.k, B.sub.k, . . . .
An equivalent circuit for the pin electrode, control electrode and recording medium is shown in FIG. 3. An H-type circuit is shown wherein the paper has three layers: a dielectric layer facing the pin electrodes, a low resistance layer and a base layer. In FIG. 3, a series circuit consisting of capacitor C.sub.1 and resistor R.sub.2 exists between the control electrode and ground. Capacitor C.sub.1 represents the electrostatic capacitance between the control electrode and the low resistance layer of the recording medium; resistor R.sub.2 represents the resistance between the low resistance layer of the recording medium and ground. Furthermore, it is considered that a series circuit consisting of capacitor C.sub.2 and resistor R.sub.3 exists between the pin electrode and ground. Capacitor C.sub.2 represents the electrostatic capacitance between the pin electrode and the low resistance layer of the recording medium; resistor R.sub.3 represents the resistance between the low resistance layer of the recording medium and ground. Resistor R.sub.1 is connected between node a, which is the connecting point of capacitor C.sub.1 and resistor R.sub.2, and node b, which is the connecting point of capacitor C.sub.2 and resistor R.sub.3. Resistor R.sub.1 represents the resistance within the low resistance layer existing between the control electrode and the pin electrode. Resistor R.sub.1 has a lower resistance value than resistor R.sub.2 or resistor R.sub.3.
In this circuit, when voltage V.sub.s (+300 V) and voltage Vn (-300 V) are supplied to the control electrode and the pin electrode respectively, the potential at point a will equal V.sub.a. When the sum of voltages Vn and V.sub.a exceeds the recording threshold voltage, recording will occur. In the event that voltage V.sub.s is a positive pulse as shown in FIG. 4(A), the potential V.sub.a will vary as shown in FIG. 4(B). That is, at the moment when voltage V.sub.s is supplied to the control electrode, potential V.sub.a will have a maximum value and then gradually diminish as capacitor C.sub.1 is charged (see FIG. 4(B)(1). At the time when voltage V.sub.s is no longer supplied to the control electrode, potential V.sub.a will decrease to a negative value. When voltage V.sub.s is again supplied to the same control electrode in order to record under the next array of pin electrodes potential V.sub.a will tend to rise from a slightly negative potential V.sub.a (off) toward the maximum value. This occurs because capacitor C.sub.1 has not been completely discharged. Accordingly, the potential at node a will be the maximum value decreased by the value of V.sub.a (-off) (FIG. 4(B)(2)). This produces a change in the recording density of each pin electrode from one moment to another. In addition, a change in recording density is produced among adjacent pin electrodes since the potential of connecting point a corresponding to an adjacent control electrode is not decreased if the voltage is supplied to that control electrode for the first time (see FIG. 4(B)(1)). Consequently, recording density will differ for pin electrodes near the control electrode which receive a supply voltage for the first time (FIG. 4(1)) and pin electrodes near the control electrode which receive a supply voltage for the second time (FIG. 4(2)), even within the same pin electrode array. As a result, irregularity of recording density will occur.
In order to improve this situation, the prior art has considered supplying voltage a second time to the same control electrode only after a sufficient time interval. However, it takes a long time to record by this method and it could not be utilized for high-speed recording. So the method as shown in Table 2 was proposed to overcome these problems.
TABLE 2 ______________________________________ Array of Recording Voltage Impressed Positons Control Electrodes ______________________________________ A.sub.1 P.sub.1 P.sub.2 A.sub.2 P.sub.3 P.sub.4 . . . . . . . . . A.sub.k P.sub.2k-1 P.sub.2k . . . . . . . . . A.sub.n P.sub.2n-1 P.sub.2n B.sub.1 P.sub.2 P.sub.3 B.sub.2 P.sub.4 P.sub.5 . . . . . . . . . B.sub.k P.sub.2k P.sub.2k+1 . . . . . . . . . B.sub.n P.sub.2n P.sub.2n+1 ______________________________________
In this method, voltages are first supplied to both control electrodes P.sub.1 and P.sub.2, and then to both control electrodes P.sub.3 and P.sub.4, and so on, successively to both control electrodes P.sub.2k-1 and P.sub.2k in order to successively record under odd pin electrode arrays A.sub.1, A.sub.2, . . . , A.sub.k, . . . , A.sub.n. Then, voltages are first supplied to both control electrodes P.sub.2 and P.sub.3, and then to both control electrodes P.sub.4 and P.sub.5, and so on, successvely to both control electrodes P.sub.2k and P.sub.2k+1 in order to successively record under even pin electrode arrays B.sub.1, B.sub.2, . . . , B.sub.k, . . . , B.sub.n. According to this method, the above-mentioned problems are improved. However, with this method, voltages V.sub.n are continuously supplied to either pin electrodes A.sub.1, A.sub.2, . . . , A.sub.k, . . . , or pin electrodes B.sub.1, B.sub.2, . . . , B.sub.k, . . . , from pin electrode control circuit 4.sub.a or 4.sub.b. Due to this continuous supply of voltage, a trailing voltage of V.sub.n occurs. This trailing voltage produces a "ghost phenomenon" during recording, so that recording occurs under a pin electrode array which should not be recording. Furthermore, due to the movement of the paper during recording and the time difference between recording by pin electrode array A.sub.k and recording by adjacent pin electrode array B.sub.k, the recorded picture is distorted.
In order to eliminate the recording distortion another prior art system was proposed. In this system, pin electrode arrays are separated into four groups: a first group of arrays A.sub.1, . . . , A.sub.k, . . . ; a second group of arrays B.sub.1, . . . , B.sub.k, . . . ; a third group of arrays C.sub.1, . . . , C.sub.k ; and a fourth group of arrays D.sub.1, . . . , D.sub.k, . . . , so that every fourth array is in the same group (A, B, C, or D). The groups are connected to pin electrode driving circuits 4a, 4b, 4c, 4d, respectively, as shown in FIG. 5 and are driven in the order shown by Table 3.
TABLE 3 ______________________________________ Array of Recording Voltage Impressed Positions Control Electrodes ______________________________________ A.sub.1 P.sub.1 P.sub.2 C.sub.1 P.sub.3 P.sub.4 A.sub.2 P.sub.5 P.sub.6 C.sub.2 P.sub.7 P.sub.8 . . . . . . . . . B.sub.1 P.sub.2 P.sub.3 D.sub.1 P.sub.4 P.sub.5 B.sub.2 P.sub.6 P.sub.7 D.sub.2 P.sub.8 P.sub.9 . . . . . . . . . ______________________________________
In this system in order to alternately record under the electrode arrays of the first and third groups A.sub.1, C.sub.1, A.sub.2, C.sub.2, . . . voltages are first supplied to both control electrodes P.sub.1 and P.sub.2, and then to both control electrodes P.sub.3 and P.sub.4, and so on, successively to control electrodes P.sub.2k-1 and P.sub.2k. In order to alternately record under the electrode arrays of the second and fourth groups B.sub.1, D.sub.1, B.sub.2, D.sub.2, . . . , voltages are first supplied to both control electrodes P.sub.2 and P.sub.3, and then to both control electrodes P.sub.4 and P.sub.5, and so on, successively to control electrodes P.sub.2k and P.sub.2k+1. According to this technique, occurrence of ghost phenomena are avoided because voltage is not successively supplied to the same pin electrode arrays. However, the use of additional pin electrode driving circuits 4c, 4d is necessary, and the existence of recorded image distortion remains.